The invention relates to a method for estimating the rotor time constant of an induction machine. More specifically, the invention relates to a method which can be used to estimate the rotor time constant during the DC magnetization period of the induction machine prior to the controlled use of the machine.
In a plurality of control methods, the essential parameters of the induction machine are needed for reliable and accurate control of the machine. These parameters include stator and rotor resistances Rs, RR and main and stray inductances LM, σLs. The rotor time constant τr is the ratio between the main inductance and the rotor resistance τr=LM/RR. Thus by estimating the rotor time constant one can calculate the main inductance if the rotor resistance is known.
There are multiple methods for carrying out the estimation of the mentioned parameters. One of these methods is disclosed in U.S. Pat. No. 5,929,400, in which the estimation is based on current injection. The estimation of rotor time constant requires an accurate estimate of stray inductance, which makes the estimation in U.S. Pat. No. 5,929,400 prone to errors. The estimation of the stray inductance always includes considerable uncertainty, because eddy currents and saturation effects cause unknown errors to the calculation.